You have another choice to consider. You could afix your processor design to have external bus interface and then wire it up in proper manner to an 8-bit ISA style connector. Then you can plug in an ISA type PC modem board. These are from the far east and relatively inexpensive. The software to communicate with an ISA modem is very straigtforward as the modems genrally emulate the register level interface of a 16C450 UART chip. If you search on the WEB you will find plenty of examples of PC based C code and Assembler code that initializes and operates 16C450 type UARTS. You can also find PC based direct modem initialize code too.

Hope this may work for you.
Michael Karas